Putting it into perspective, selling 2 million first week is incredibly rare. Only 70 games have done 2 million lifetime in Japan, so doing it in four days is quite a feat.
To my knowledge, Pokemon Black/White holds the record for highest opening week in Japan, at 2.6 million. No other Pokemon game has managed over 2 million opening week.
The second place for opening week is, again, to my knowledge, Dragon Quest 9, with 2.35 million.
The third place belongs to Final Fantasy 7, with 2.025 million, and the fourth biggest opening of all time is Monster Hunter Freedom 3 with just shy of 2 million.
If Monster Hunter 4 did indeed pass 2 million copies sold in the first week, that'll make it (most likely) the third biggest opener of all time in Japan. The link talks about shipments, though, so it's probably not going to be over 2 million sold unless it completely sells out. Still, near 2 million is simply fantastic.
Monster Hunter now really cements its place as one of the current four big franchises in Japan, alongside Pokemon, Mario and Dragon Quest. Three games (Freedom Unite, Freedom 3, Tri (all platforms) selling 4 million or more is something only Pokemon has managed.
Pokemon has 6 games over 4 million, Mario has 5, Brain Training has 2, Dragon Quest has 2, Animal Crossing has 1 (likely to become 2 as New Leaf passes 4m) and Tetris has one. Soon, Monster Hunter is likely to have 4. That's just huge, all released in a 5 year period. That's simply huge.
